Polypropylene is commonly used in the packaging industry due to its excellent balance of properties and cost. One of the most common methods of manufacture using polypropylene is the extrusion process, in which the polyolefin is melted in an extruder well in excess of its melting point, and passed through a die which shapes the polymer into a flat sheet, a hollow tube, etc for formation into a final part. In this extrusion process there are several challenges. One of the challenges experienced in the extrusion of polypropylene is a condition called “plate-out”. “Plate-out” refers to the undesirable deposition of additives or low molecular weight polymers upon processing equipment during the processing of polypropylene containing additives. Such additives could include nucleating agents, clarifiers, slip agents, anti-static agents, acid scavengers, colorants, ultraviolet light absorbers and perhaps others. Such additives are compounded into the polypropylene before the production of a plastic part.
In general, it is desirable to avoid “plate-out”. One reason is that “plate-out” causes manufacturing malfunctions, quality problems and downtime. This may occur during the sheet extrusion process when a plastic sheet is being formed between multiple polishing rolls. This undesirable “plate-out” of material upon a polishing roll can cause the polyolefin sheet not to remove itself as it should from the forming roll. When the sheet improperly adheres to the roll, it can wrap around the roll, disrupting manufacturing operations and causing downtime. This situation can be costly and difficult. Also, the build-up of “plate-out” on the polishing rolls can degrade the optical properties of the sheet, most notably surface gloss.
Various techniques have been applied to control or reduce “plate-out”, but many such techniques have been only partially successful. This invention is directed toward a method that is capable of eliminating the problems associated with “plate-out” in the formation of plastic parts.
U.S. Pat. No. 4,251,407 to Schroeder et al. is directed to the use of finely divided zinc oxide in polypropylene. The zinc oxide is stated to act as an acid acceptor, and is believed to interfere with migration of low molecular weight polymer to the surface of the polymer melt, thereby preventing the “plate-out” of such material on the processing equipment. One disadvantage of the process shown in the Schroeder patent is that zinc oxide negatively impacts the clarity of the plastic parts produced in the process. Therefore, although zinc oxide may provide certain “plate-out” advantages, those advantages may be outweighed by the undesirably low clarity of the plastic parts made, rendering it unusable in systems with clarifying agents. Moreover, other solid particulates are known to function similarly, including talc and calcium carbonate, though they also will negatively affect the clarity performance of a polyolefin part.
It is commonly known in the polypropylene industry that the addition of lubricants, such as glycerol monostearate (GMS) or calcium stearate, in small amounts (1000 ppm and lower) can reduce “plate-out” by reducing shear internally and lowering the amount of low molecular weight polymer and volatile additives driven off during the plastification process. While somewhat effective, GMS and calcium stearate, as well as other lubricants in this class, can not completely eliminate plate-out, and if used at too high a level can actually make plate-out worse. These molecules also tend to migrate through the plastic in the final part, depositing on the surface of the part and creating a white film, referred to as bloom. This is an undesirable side-effect of this method.
Fluoropolymers such as Dynamar (sold commercially by Dyneon) and Viton Free Flow (sold commercially by DupontDow Elastomers) are highly effective in reducing plate-out in polyolefin film extrusion. These additives, as written by Bossche et al. migrate through the polymer to the polymer-metal interface, creating a layer that favors polymer slip. This layer allows for reduced shear and reduced processing temperatures, thereby reducing migration of additives from the polymer melt. Furthermore, these additives reduce the surface energy of the die wall, reducing the affinity of the additives and degraded polymer for the metal surface. These additives, while effective in films, tend to be expensive and adversely affect clarity in thicker parts.
It would be desirable to develop processes, compounds, or compositions that are capable of reducing or eliminating “plate-out,” while not negatively impacting the transparency of the given polyolefin article in the initial or post-manufactured state. The present invention is aimed at such a system.